The modification of electronic structures of manganese oxides due to lithium intercalation has been investigated by the DV-X alpha molecular orbital method. In this study the process of lithium intercalation is assumed to be divided into two steps: the Lithium occupancy (chemical effect) and the lattice expansion (structural effect). By the lithium occupancy the energy gap lying between the O-2p and the Mn-3d bands increases, and there is attendant increase in the ionic character in the chemical bonding. For example, the ionicity of the oxygen becomes more negative whereas that of the manganese becomes more positive. However, by the lattice expansion, the energy gap decreases and the electronic state recovers to some extent to the original state in MnO2. In particular, it is noted that the ionicity of the manganese in LiMn2O4 is fully recovered to the value in MnO2, indicating that lithium intercalation reduces the oxygen ion, but not manganese ion.